1. Field of the Invention
The present invention relates to a freeze drying apparatus including a vacuum chamber and an evacuation system.
2. Background of the Art
Freeze drying is a technique for conserving temperature sensitive commodities whose characteristics are to be retained. Freeze drying is used primarily in pharmacology, biology and medicine.
The usual freeze drying process includes a primary drying process and a supplemental drying process which takes place approximately as follows. After freezing a commodity which contains water, the water now present as ice is sublimed under vacuum, typically at approximately 10.sup.-1 mbar. After this primary drying process, a supplemental drying process takes place during which adsorptively bound moisture is removed to realize an extremely low residual moisture content. During this subsequent supplemental drying, the product is heated within permissible temperature limits determined by the nature of the commodity. The pressure at which the supplemental drying takes place lies at about 10.sup.-3 mbar. Examples for permissible temperatures commodities include 40.degree. C. for biological products.
Since freeze drying is performed in charges, it is desirable to make the individual charges as large as possible and the monetary value per charge is thus correspondingly high. At present it has become quite customary to operate with charges whose value lies over 100,000 German Marks, i.e., approximately $175,000. The loss of a charge due to an apparatus malfunction thus leads to significant monetary losses.
Apparatus malfunctions that endanger a commodity charge include malfunctioning of the compressor refrigeration machines which supply coolant to the commodity support surfaces in the vacuum chamber and the condensation surfaces in the condenser. Such malfunctioning may involve a defect in the coolant compressor itself or a loss of coolant from the refrigeration circuit, complete or partial loss of current, loss of cooling of the liquefiers in the coolant circuit of the coolant compressors if, for example, the water supply is lost, or loss of system control.
All of these possible malfunctions result in the loss of compressor cooling and thus loss of pumping power in the evacuation system. Since it is necessary to supply the energy required for the sublimation of the water in the commodity to be dried in the form of heat, loss of the evacuation system results in the commodity heating up since the water vapor escaping from the commodity is no longer pumped away. This, together with a rise in pressure in the freeze drying chamber, causes the commodity to begin to thaw. This circumstance results in a loss of quality, particularly for highly sensitive medicinal products, and the possible loss of the entire commodity charge.